1. Field of the Invention
The present invention relates to a device and method for adjusting currents of lamp tubes.
2. Description of the Related Art
Display devices like CRT (Cathode Ray Tube) or LED (Light Emitting Diode) displays have so large a volume and power consumption that they cannot efficiently achieve user requirements. However, LCDs (liquid crystal displays) have advantages of not only smaller volume and lower power consumption, but also low radiation. LCDs have superior contrast and less indistinct moving pictures, thus LCDs better fit user requirements. Concurrently, the size of one LCD and the brightness of a backlight module for the LCD have been increased, but the CCFL (Cold Cathode Fluorescent Lamp) in the backlight module has a current saturation problem; that is, the brightness of the CCFL can not be increased only by increasing the current thereof. Thus, adding lamp tubes will be a better solution to achieve a higher brightness of the backlight module.
FIG. 1 shows a block diagram of a circuit in a backlight module with two lamp tubes. A feedback device 17 monitors currents I14a and I14b of a first lamp tube 14a and a second lamp tube 14b respectively, and then outputs a feedback signal S1 to a main control device 10. The main control device 10 outputs a driving voltage to a resonance circuit 11 according to the feedback signal S1, and then the resonance circuit 11 outputs a voltage-control signal. A transformer 12 increases the amplitude of the voltage-control signal to drive the first lamp tube 14a and the second lamp tube 14b. The first lamp tube 14a and the second lamp tube 14b shall have equal currents so as to produce equal brightness thereof. A first capacitance 13a and a second capacitance 13b are used to isolate DC voltage parts of the voltages of the first lamp tube 14a and the second lamp tube 14b. However, impedance characteristics of the first lamp tube 14a and the second tube lamp tube 14b are different from each other, and equivalent impedances of the first capacitance 13a and the second capacitance 13b have large errors. Thus, the currents 114a and 114b are not the same, and brightness of the first lamp tube 14a is not equal to that of the second lamp tube 14b. Therefore Resultantly, the brightness of the LCD is not uniform and lifetime of each lamp tube is decreased.
FIG. 2 shows a block diagram of a conventional circuit solving the problem of unequal currents. Feedback devices 27a and 27b monitor the current I24a of the first lamp tube 24a and the current I24b of the second lamp tube 24b respectively. Main control devices 20a and 20b receive feedback signals Sa1 and Sa2 and then adjust the currents I24a and I24b accordingly. The currents I24a and I24b are unequal because devices in each circuit for these lamp tube 24a and 24b have errors and different feedback points, and then brightness of the first lamp tube 24a is still not equal to that of the second lamp tube 24b. Thus the conventional circuit cannot entirely solve the problem of unequal currents, and, additionally requires a large number of additional devices in the circuits, increasing cost and possible errors of the circuits.